1. Field of the Invention
The invention relates to a device for the transmission of data using the idle time on a telephone line connecting the switching system of a telephone exchange and the telephone in a user's home.
2. Description of the Prior Art
Telephone communication normally involves the connection of the switching system of a telephone exchange and a user's telephone through the medium of a telephone line. When the telephone receiver is lifted off the hook, a loop is formed through the telephone line between the telephone and the exchange's switching system making voice communication possible by the transmission of signals around said loop. There are known data transmission devices which make use of telephone line idle time For the unidirectional transmission of broadcast data in the Form of, for example, music to a user's home. A data transmission device of this type calls for the installation in a telephone exchange of an exchange device which can be used to switch the telephone line selectively between the switching system of the exchange itself and a center unit which is installed in an independent center and which is used for the transmission of analog signals in the form of speech or music. Each user's home is fitted with a home unit with a speaker, which is connected to the telephone line and which, when the receiver is lifted off the hook, connects the exchange device to the exchange's switching system to enable voice communication and, when the receiver is placed on the hook, switches the exchange device to said center unit which transmits broadcast signals down the telephone line to each home unit. In practice, a plurality of center units are normally connected to a single exchange device which can be turned on and off or switched selectively between said center units (in order to change channels) by the remote operation of each home unit. In addition to the speech and music signals referred to above, a center unit can also be used to broadcast data such as character or image data. In this sort of case, an image device would be used as the home unit.
However, although a conventional data transmission device normally permits the selection of any one of a plurality of possible broadcast signals, once this choice has been made, transmission is limited to the selected signal only. In other words, if, for example, two different categories of broadcast signals such as music and image signals were to be transmitted at the same time to a user's home, it would be impossible to coordinate the output of these signals at the user's end and this in turn makes it difficult to meet the modern demand for increased data transmission.